The present invention relates to equipment for reducing wood and more particularly to wood reduction machines for chipping or grinding scrap timber, limbs, brush and other wood waste.
There is a wide variety of machines available on the market for reducing waste wood, such as scrap timber, tree limbs and brush. The two most common types of wood reduction machines are chippers and grinders. As the names imply, chippers reduce wood by cutting it into wood chips using a set of chipper knives and grinders operate by essentially hammering wood into wood fragments using a hammermill.
The type of wood reduction equipment used in a given situation is often dictated by the character of the wood waste. On the market, wood chips typically bring a premium over ground wood. However, not all wood waste is suitable for the production of wood chips. With lower quality wood waste that may include a substantial amount of sand, gravel and other contaminants, it may be desirable to use a wood grinder. The hammers used in wood grinders typically have a greater ability to withstand the contaminants than the knives contained in wood chippers. As a result, it may be desirable to grind lower quality wood waste to avoid the excess wear that might occur during chipping. Given the premium enjoyed by wood chips, higher quality wood waste is often reduced using a wood chipper. Wood reduction equipment can be rather expensive, and many wood waste processors may not be able to afford both wood grinders and wood chippers.
With the continued push toward renewable resources and recycling, there has been a growth in the demand for wood fuel sources. Reduced wood waste is an ideal wood fuel source for many wood fuel applications. Although lower quality wood waste may be used as a fuel source, ground wood waste can present problems for wood fuel handling systems. For example, large consumers of wood-based fuels will often include pneumatic feed systems for conveying wood fuel from a supply center to the wood burner. Many conventional pneumatic feed systems do not work as well with ground wood waste, presumably because it has a greater tendency than wood chips to cling or clump together. Many wood processors that process lower quality wood waste have purchased wood grinders to avoid the excess wear that might accompany wood chippers. Because of the increasing demand for wood chips produced from lower quality wood waste, these processors might wish to be able to at least occasionally produce wood chips. However, the cost of adding a wood chipper to permit occasion use may be cost prohibitive. Accordingly, there is an increased need to allow existing wood grinders to be at least temporarily converted into chippers for reducing wood waste into wood chips.
At least one wood grinder (or wood hog) available on the market is available with interchangeable chipper attachments that allow its hammermill to be converted into a chipping mill. This wood grinder includes a generally conventional stacked-plate rotor in which hammer inserts are secured in annular channels in the rotor by a plurality of rotor pins. To convert to a wood chipper, the hammer inserts are removed and replaced with knife inserts. The knife inserts are spaced apart from one another around the rotor, and are mounted in the annular recesses on the rotor pins in essentially the same manner as the hammer inserts. The knife inserts are similar in width to the hammers inserts. The design and configuration of the knives is such that there the converted chipper mill has relatively large spaces between the knives and the knife holders. In fact, significant portions of the rotor plates are exposed to the wood waste during wood reduction. Although the knife inserts allow the wood grinder to be converted into a chipper, the system has inherent limitations that may affect chip quality and may lead to inconsistent chip size. For example, because of the open spaces between and around the knives, the chipper mill has relatively large dead spaces and provides relatively little control over chip size and chip quality.